Performance characteristics of optical fibers can be degraded by environmental elements. For example, the interaction of water with the surface of a silica fiber produces surface modifications which can reduce the strength of the fiber. Any small flaws on the surface can reduce the strength of the fiber and provide a site for moisture-induced stress-corrosion crack growth, which can lead to ultimate fiber failure. Also over a period of time, hydrogen, which can be present at the fiber surface at levels orders of magnitude higher than those found in the atmosphere, can diffuse into an optical fiber and increase the optical loss in a signal carried by that optical fiber.
In order to prevent such interactions, a hermetic coating is applied to the fiber for preventing deleterious environmental elements (typically, water and/or hydrogen) from interacting with the fiber. Such a coating acts as an impermeable hermetic barrier between the fiber and the environment. One such coating, e.g., a carbon coating, is applied under stable ambient conditions to the outer surface of a silica cladding of the fiber by inducing decomposition of a suitable carbon containing organic precursor gas, e.g., acetylene, to form a thin carbon film on the fiber surface, as described by F. V. DiMarcello et al., in a U.S. patent application, Ser. No. 098,253, filed Sept. 18, 1987. Thickness of the carbon coating being applied to the fiber is measured by moving the optical fiber through an electromagnetic field so that the conductive carbon coating interacts with the electromagnetic field, as described by R. M. Atkins et al., in a U.S. patent application, Ser. No. 387,261, filed July 31, 1989, and incorporated herein by reference. The optical fiber thereafter typically is passed through an ultraviolet light curable liquid material and an ultraviolet light for transforming the material into a solid polymeric jacket on the optical fiber.
Fibers processed as described above are sometimes performance-limited in certain applications because ultraviolet light curable polymers typically breakdown at temperatures which are too low for some high temperature environments.
Alternative polymeric materials are a group of heat-curable polymers, which typically break down at temperatures high enough for many high temperature environments. In the prior art, such heat-curable polymers are heated by moving the heat-curable liquid, coated on the optical fiber, through an oven. The heat-curable liquid is heated by convection from the hot air in the oven. Under conditions of rapid heating, the polymer starts cross-linking, or cures, starting at the surface nearest the source of heat, and a film forms on the outer surface of the polymer coating. Continued heating causes bubbles to form in the remaining liquid material due to (1) thermal driven release of dissolved gases, (2) volatilization of components comprising the resin, or (3) volumetric changes in the coating material brought about by the thermally driven cross-linking activity. When the heating is rapid, those bubbles are trapped in the polymer because of the solid film which initially formed on the outer surface of the polymer and the inner surface being blocked by the cladding of the optical fiber. As more of the liquid material cross-links, the bubbles are permanently trapped in the solid polymer forming undesirable defects in the desired solid polymeric coating and causing increased microbending loss and/or a reduction of reliability due to reduced coverage of the silica.